Diversity of morphology of type II spicules in MURaM-ChE simulations

Abstract

Spicules are ubiquitous, small-scale features in the solar atmosphere, exhibiting a jet-like appearance most clearly identified by their apparent motion in off-limb observations. While they are often interpreted as narrow, thread-like structures, their true three-dimensional (3D) structure remains unknown. We aim to uncover the 3D morphology and dynamics of fast-evolving spicules (type II) using a MURaM-ChE simulation. We use a H$α$ proxy that has been developed using non-equilibrium (NE) hydrogen populations in MURaM-ChE. The proxy, modelled as an escape probability, is synthesised to isolate on-disc as well as off-limb H$α$ wing features. The 3D structure of these features is investigated using the 3D information on opacity in H$α$. We identify type II spicules, with unique 3D morphologies: the dominant being thread-like and slab-like. The appearance of spicules as slabs or threads is a function of time and Doppler velocity. The spicules extending above the spicule-forest (2Mm -3Mm above the surface) tend to be located at quasi-separatrix layers (QSLs). We find that the spatially resolved contributions to the opacity of spicules are often similar for spicules synthesised in the horizontal direction, and their on-disk rapid blueshifted excursion (RBE) synthesised in the vertical direction at the same Doppler velocity of 37km/s. This confirms that RBEs are indeed the on-disc counterparts of spicules. Furthermore, our analysis indicates that cross-field motions can significantly contribute to spicule dynamics. Spicules exhibit a range of morphologies, including both slab-like and thread-like structures. Their observed appearance depends strongly on line-of-sight projection and Doppler sampling. Spicules are preferentially located at QSLs, highlighting the role of magnetic topology in driving spicular dynamics.